Production of carbon fiber starts with the manufacture of the so called precursor fiber. The ideal requirements for a carbon fiber precursor are that it should be easily converted to carbon fiber, give a high carbon yield and allowed to be processed economically. Other important requirements for precursor materials are spinnability i.e. the ability to form filaments, stretchability, i.e. the ability to stretch and align its molecular structure along the fiber direction and the ability to form a thermoset in the stabilization process i.e. to become infusible so that the individual filaments do not stick together. The sticking of filaments must be avoided because it causes surface defects and unequal diffusion during thermal conversion to carbon fiber leading to poor mechanical performance. The material must furthermore be able to maintain its stretched structure during thermal conversion to form linearly oriented graphite structures in the carbon fiber. Only few materials exhibit this combination of requirements.
About ninety-five percent of all carbon fibers are produced from poly-acrylonitrile (PAN). PAN has a continuous carbon backbone and nitrile groups that are ideally positioned for cyclization to occur, enabling the formation of a ladder polymer during stabilization, as a first stage towards the carbon structure of the final CF. The ability of PAN to maintain its oriented structure during stabilization is the key for PAN-based carbon fiber's excellent strength and stiffness. Carbon fiber out performs most other materials such as metals or glass fiber regarding mechanical properties but its high price is regarded as the main bottleneck that hinders its penetration into mass markets (such as automotive and wind energy). The high price is due to the high manufacturing costs of carbon fiber and in case of PAN-based carbon fiber the cost of the PAN precursor contributes to more than 50% to the total costs [1]. This motivates the development of alternative low-cost precursor concepts.
Precursors from melt-spun lignin and lignin derivatives and hybrid precursors from solvent-spun PAN and lignin (WO 2012003070 A1) are two important technology platforms under development. Typical target properties for these kinds of carbon fibers are 170 GPa tensile modulus and 1.7 GPa tensile strength [2], i.e. lower than the properties of commercial PAN-based carbon fibers. Lignin is a polyaromatic polyol and constitutes, after cellulose, the second largest material component in wood and other lignocellulosic plants. The amount of carbon in lignin is relatively high, approx. 60-65% and therefore lignin is regarded as a promising raw material for carbon fiber with respect to giving a high carbon yield. During chemical pulping cellulosic fibers are separated from softwoods, hardwoods, and annual plant biomass, for further processing to paper, board, tissue products and man-made cellulose fibers. Kraft pulping is the dominant chemical pulping process. Other processes include soda pulping, sulfite pulping (which gives lignosulfonates) and the organosolv process. In alkaline pulping (i.e. kraft and soda pulping), large quantities of lignin become dissolved in the alkaline pulping liquor, known as black liquor, a highly alkaline complex mixture containing used cooking chemicals, solubilized wood lignin, carbohydrates and organic acids. From there the lignin can be further processed to energy by combustion of the partly evaporated black liquor or, alternatively, be isolated, for example by precipitation using acid. The chemical structure of precipitated lignin is determined by the type of biomass used and the pulping method. Lignin may be melt-spun to filaments but such lignin fibers may have several disadvantages for usage as carbon fiber precursors. Lignin fibers easily melt during thermal conversion to carbon fiber. In order to achieve a thermoset material, i.e. an infusible non-sticky stabilized fiber for conversion into the final carbon fiber, the lignin precursor has to be stabilized at extremely low heating rates. Values are reported in the range between 0.05° C./min [3] up to 0.5° C./min [4]. Consequently, the total residence times during stabilization for reaching the final stabilization temperature of typically 250° C., range from 7.5 h to 75 h (see table 1). Such long stabilization times hamper the competitiveness of lignin-based carbon fibers. For comparison, PAN precursors are exposed to stabilization times of about 1.5 hours. Another report states stabilization rates for monofilament fibers from low-molecular weight softwood lignin of 15° C./min [5]. The stickiness of a multifilament yarn during stabilization was not studied in that report. Fibers were interpreted as fully stabilized when they no longer showed any glass transition point as measured by differential scanning calorimetry or melting point under thermal treatment in a melting point microscope. The carbonization rate was 3° C./min up to the final carbonization temperature of 1000° C./min, leading to a total carbonization time of 250 minutes, i.e. far longer than the carbonization times for commercial carbon fiber manufacture that is in the range of a few minutes.
WO 2012003070 describes a method for the manufacture of dopes containing poly-acrylonitrile (PAN) and lignin for the production of carbon fiber precursors via solvent spinning. Also for this concept, filament stickiness during thermal conversion of the precursor to carbon fiber has been reported [2].
Lignin precursor fibers are characterized by an extremely brittle behavior and low mechanical properties (30 to 35 MPa tensile strength and 0.5 to 1% elongations-at-break) throughout the whole temperature range between room temperature and 300° C. [6]. The literature does not report on a successful continuous conversion of lignin precursors to carbon fiber, only batch-wise conversion. The most probable explanation is that lignin fibers cannot withstand the mechanical stresses during continuous production caused by fiber transportation (via rollers), stretching and winding/unwinding. Cellulose-precursors, by contrast, have higher mechanical performance. The subsequent stabilization of the cellulose precursor, however, is associated with a very high yield loss and depolymerization. Two competing reactions occur at 250-300° C. The desired dehydration of cellulose chains and the unwanted generation of levoglucosan and simultaneous depolymerization [7]. The stabilized cellulose fiber is much weaker than the original precursor [8 (p. 15)], [9] and cannot be stretched during stabilization. Stretch graphitization at temperatures between 2500° C. and 3000° C. must be applied in order to form oriented graphite domains for high mechanical performance [8] which causes a poor material yield (typically 10-20%). The high temperatures needed and the poor material yield lead to high production costs for cellulose-based carbon fiber.
The industrial production of carbon fibers started in 1963. At the time C. E. Ford and C. V. Mitchell from Union Carbide developed and patented a continuous method for manufacturing carbon fibers from cellulosic man-made precursors [10]. In 1964, carbon fibers with the trade name »Thornel 25« having strengths of 1.25 GPa and moduli of 172 GPa were introduced to the market. Later on »Thornel 50«, »Thornel 75«, and »Thornel 100« followed. The latter had strengths of 4.0 GPa and moduli of 690 GPa. This excellent property profile could however only be obtained through stretch-graphitization at temperatures between 2500° C. and 3000° C. Only at these high temperatures, a plastic deformation is possible for a cellulose-based precursor yielding high orientations and thus a carbon fiber with competitive mechanical properties. This manufacturing process was costly and associated with low carbon yields in the range between 10 and 20%. This led to the complete cessation of carbon fiber manufacture from cellulosic man-made precursors for reinforcing applications. This comedown is closely related to the development of PAN-based carbon fibers which can be manufactured in a more cost-competitive way with similar mechanical properties. Without the expensive stretch graphitization step carbon fibers based on cellulose-based precursors attain only inferior mechanical properties with moduli in the range of 40 GPa [9] to 97 GPa [11] and tensile strength from 160 MPa [12] to 1.1 GPa [11] preventing this kind of fiber from being competitive for structural applications.
A further method for the manufacture of carbon fiber is disclosed in DE1952388.
Also in US20150078983 a method for the manufacture of a carbon fiber is disclosed which involves cellulose fibers.
Cellulose is a lower cost raw material than PAN and, in contrast to PAN, a renewable raw material. However, the high yield loss and the high energy input needed during stretch-graphitization of cellulose make the manufacture of cellulose-based carbon fiber not competitive.
The present invention addresses the problems of carbon fiber production that are related to stretching and generation of orientation. Thus, the problem to be solved is to convert cellulose-based precursor cost efficiently to high-performing carbon fiber or precursors thereof.
It has now surprisingly been found, that by using a certain method, involving the addition of lignin or lignin derivate to the cellulose, a stabilization step and a stretch-pre-carbonization step, a highly-oriented intermediate carbon fiber can be provided. Said stabilization step also give rise to a thermally stabilized, non-sticky and stretchable fiber. Said intermediate carbon fiber is provided with such properties that alleviate or even solve one or more of the problems connected with the manufacturing of carbon fibers from cellulose. Said intermediate carbon fiber can be converted to a highly-oriented carbon fiber. Furthermore, said carbon fiber is completely based upon renewable resources.